1. Field of the Invention
This invention relates to a stabilized polymer processing additive system, to thermoplastic polymers having improved extrusion characteristics that employ the stabilized system, and to an extrusion process that employs the additive system.
2. Background of the Art
Extrusion of polymeric materials in the formation and shaping of articles is a major segment of the plastic or polymeric articles industry. Various materials from fibers, filaments, films, sheeting, tubes, structural elements, ducts, inserts, layered articles, and other articles having a defined cross-section can be readily and inexpensively made by extrusion processes. The fundamental extrusion process requires that a material in a fluid or fluidizable state is forced through an outlet and that the material is then converted into a non-fluid state. When the conversion from a fluid to a non-fluid state is performed in a sufficiently rapid time (with respect to the ability of the extruded material to maintain its general shape and appearance), the non-fluid article will retain a cross-section shape with the appearance of the edges of the outlet. During extrusion processes, one of the critical areas of interaction which may control quality of the article and performance of the extrusion process is the interaction of the fluid material with the extrusion outlet, often in the form of a slit, hole, opening, or other shaped outlet. The structural element which provides the physical outlet is usually referred to as a die or die head.
Westover, R. F., "Melt Extrusion", Encyclopedia of Polymer Science and Technology, Vol. 8, John Wiley & Sons, (1968) pp. 573-581 states that for any polymer there is a certain critical shear rate above which the surface of the extrudate becomes rough and below which the extrudate will be smooth. He further states that in order to achieve the highest possible flow rate from the extruder and to achieve the most uniform extrudate cross section the processor must control extrudate roughness or distortion. Some of the various types of extrudate roughness and distortion observed in high and low density polyethylenes are described in Rudin, A., Worm, A. T., Blacklock J. E., "Fluorocarbon Elastomer Aids Polyolefin Extrusion," Plastics Engineering, March 1986, pp. 63-66. Rudin et al. state that for a given set of processing conditions and die geometry, a critical shear stress exists above which polyolefins like linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and polypropylene suffer from melt defects. At low shear rates, defects may take the form of "sharkskin", a loss of surface gloss, which in more serious manifestations, appears as ridges running more or less transverse to the extrusion direction. At higher shear rate the extrudate can undergo "continuous melt fracture" becoming grossly distorted. At rates lower than those at which continuous melt fracture is first observed, LLDPE and HDPE can also suffer from "cyclic melt fracture", in which the extrudate surface varies from smooth to rough. These types of problems may also occur in any other class of extrudable polymer besides polyolefins, including, but not limited to polyacrylates, polyamides, polycarbonates, polyvinyl resins (polyvinyl chloride, polyvinylidene chloride, polyvinyl esters, polyvinyl ethers, polyvinyl alcohol, and copolymers thereof), polytetrafluorethylene, polyesters, and the like, including copolymers thereof. The authors (Rudin, A., Worm, A. T., Blacklock J. E.) state that lowering the shear stress by adjusting the processing conditions or changing the die can avoid these defects to a certain extent, but not without creating a whole new set of problems. For example, extrusion at a higher temperature can result in weaker bubble walls in tubular film extrusion, and a wider die gap can affect film orientation. The authors state that the use of fluorocarbon elastomer processing aids can permit the operation of extruders with narrower die gaps and lower melt temperatures. Others have also described the use of fluorocarbon elastomers as processing aids, see for example, De Smedt, C. and Nam, S., "The Processing Benefits of Fluoroelastomer Application in LLDPE," Plastics and Rubber Processing and Applications, 8, No. 1, (1987), pp. 11-16; U.S. Pat. Nos. 3,125,547 (Blatz), and 4,581,406 (Hedberg et al).
The use of polyethylene glycol as an extrusion processing aid has been described. For example, U.S. Pat. No. 4,013,622 (DeJuneas et al.) discloses the use of polyethylene glycol to reduce the incidence of breakdown of polyethylene in the extruder, and Canadian Pat. No. 961,998 (Hancock et al.) discloses the use of anti-oxidant-stabilized, polyolefin-based film extrusion compounds and polyalkylene glycol to prevent gel streak formation during extrusion.
U.S. Pat. No. 5,015,693 (Duchesne et al) provides an extrudable composition comprising
(A) thermoplastic hydrocarbon polymer, e.g., polyethylene, as the major or predominant component of the composition, PA1 (B) poly(oxyalkylene) polymer, and PA1 (C) fluorocarbon polymer. PA1 (i) a fluorocarbon polymer, PA1 (ii) a poly(oxyalkylene) polymer, PA1 (iii) magnesium oxide, and, optionally PA1 (iv) a stabilizer, e.g., one or more antioxidants. PA1 (A) an extrudable polymer (e.g., a predominant amount, such as at least 50% by weight of solids), and PA1 (B) an effective amount (e.g., up to 50% by weight) of the extrusion additive. PA1 (A) providing a predominant of an extrudable thermoplastic polymer and an effective amount of the extrusion additive, PA1 (B) admixing the extrusion additive with the extrudable thermoplastic polymer, and PA1 (C) extruding the admixture.
The poly(oxyalkylene) polymer and the fluorocarbon polymer are present in the extrudable composition in such relative proportions and at concentrations which, in combination or in concert, are sufficient to reduce melt defects, i.e., sharkskin, continuous melt fracture and cyclic melt fracture.
U.S. Pat. No. 5,459,187 (Goyal et al) describes polyolefin compositions having good extrusion characteristics comprising a fluoropolymer, one or more of a low molecular weight C.sub.1-4 alkyl ethers of a poly-C.sub.2-4 alkylene oxide, and a metal oxide, the weight ratio of the ether to the fluoropolymer being less than 1:1. The diclosed metal oxides on column 3 include a weak metal base comprising a metal oxide of an alkaline earth or transition metal or hydrotalcite (Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3 -4H.sub.2 O). No clearly stated purpose is disclosed for the use of the metal oxide or hydrotalcite.
The extrusion additives are often incorporated into thermoplastic hydrocarbon polymers by forming a master batch of the two. Such master batches are often prepared at relatively high temperatures under aerobic conditions. This can result in degradation of one or more of the components of the master batch and a resulting loss of efficiency of the processing additive.
It has been found that the additives disclosed in U.S. Pat. No. 5,459,187, do not stabilize the extrudable hydrocarbon polymer composition against oxidative degradation as shown hereinafter.